I just replaced a busted motherboard on my Windows 7 computer, and it no longer sleeps as it had before. With my old motherboard, whenever I put the computer to sleep, the screen faded to black and then the computer turned off after a bit. Then, when I turned it back on, it went straight to the login screen. This felt more akin to hibernation, except that the computer was able to resume right away.

With my new motherboard, going to sleep does not turn off the computer. The screen goes black as before, and the computer resumes right away when I press the power button, but the power supply keeps running at full force. The hard drive and other components turn off, but the computer does not.

What explains this difference in behavior? Is this a motherboard/BIOS feature, or is something broken?

Are you sure that the power supply keeps delivering energy?
I assume that it is just the fan that spins, which would mean that you old board had a feature to switch the power supplies fan off and you newer one does not.